In recent years a high demand has developed for ever smaller, space-saving, light and cost-effective display modules and displays for fast and adequate visualization of data. In the field of flat screens for notebooks, mobile telephones and digital cameras, LCDs (liquid crystal displays) are still predominant at the present time. However, they have some disadvantages, such as the great angular dependence of contrast and colours, slow response times in the event of picture and contrast change, and a low efficiency caused by a multiplicity of filters and polarizers, so that comparatively high energies have to be used for obtaining the required luminous intensity. In this respect, there is a high demand for small high-resolution and power-saving screens with improved presentation properties. Displays based on organic light-emitting diodes (OLEDs) represent an alternative to LCDs since they themselves comprise light-emitting pixels and, consequently, do not have any backlighting. They can be produced in the form of a film, for example, flexibly and thin with low production costs and can be operated with a comparatively low expenditure of energy. With their low operating voltage, the high energy efficiency and also the possibility of producing planar emitting components for the emission of any desired colours, OLEDs are also suitable for application in illumination elements.
OLEDs are based on the principle of electroluminescence, in which electron-hole pairs, so-called excitons, recombine with emission of light. For this purpose, the OLED is constructed in the form of a sandwich structure in which at least one organic film is arranged as active material between two electrodes, positive and negative charge carriers being injected into the organic material, a charge transport of holes or electrons to a recombination zone in the organic layer taking place, where a recombination of the charge carriers to form singlet and/or triplet excitons occurs. The subsequent radiative decomposition of the excitons causes the emission of visible useful light that is emitted by the light-emitting diode. In order that this light can leave the component, at least one of the electrodes must be transparent. This transparent electrode is generally composed of conductive oxides, referred to as TCOs (transparent conductive oxides). The starting point in the production of an OLED is a substrate, to which the individual layers of the OLED are applied. If the electrode nearest to the substrate is transparent, the component is referred to as a “bottom emission OLED”; if the other electrode is embodied in transparent fashion, the component is referred to as a “top emission OLED”. The same applies to the case of fully transparent OLEDs, in which both the electrode between the substrate and the at least one organic layer and the electrode located at a distance from the substrate are embodied in transparent fashion.
As explained, light is generated in the active zone or emission zone of the component through radiative recombination of electrons and defect electrons (holes) by means of excitonic states. The different layers of the OLEDs, for example the transparent electrodes and the at least one organic layer, generally have a different refractive index, which is naturally greater than 1. In this respect, not all of the photons generated can leave the component and be perceived as light since total reflections can occur at the various interfaces within the component or between the component and air. Moreover, a part of the light generated is also absorbed again within the component. Depending on the design of the OLEDs, besides the propagation of external modes, on account of the total reflection described above, optical substrate and/or organic modes form (that is to say propagation of light in the substrate, the transparent electrode and/or the at least one organic layer). If the electrode nearest to the substrate is not transparent (top emission OLED), besides external modes it is merely possible for modes to propagate in the at least one organic layer and/or the electrode located at a distance from the substrate, which are referred to jointly as organic modes. Only the external optical modes can be perceived as light by the observer, that proportion of the total luminescence generated within the component which is made up by said modes being approximately 20%, depending on the design of the OLED. In this respect, there is a need to couple these internal optical modes, that is to say organic and, if appropriate, substrate modes, out of the component to a greater extent in order to achieve a highest possible efficiency of the organic light-emitting component.
In order to improve the coupling-out efficiency, a multiplicity of methods and configurations, particularly for bottom emitting OLEDs, are known which are concerned with the coupling-out of the optical substrate modes. For this purpose, the article “30% external quantum efficiency from surface textured, thin-film light-emitting diodes” by I. Schnitzer, Appl. Phys. Lett., Volume 63, page 2174 (1993), proposes roughening the surface of the substrate, whereby the occurrence of total reflection at the interface between substrate and air is avoided to a considerable extent. This roughening may be achieved for example by etching or sandblasting the substrate area remote from the organic portion. The paper “Improvement of output coupling efficiency of organic light-emitting diodes by backside substrate modification”, by C. F. Madigan, Appl. Phys. Lett., Volume 76, page 1650 (2000), describes applying a spherical pattern to the rear side of the substrate surface. Said pattern may comprise an array of lenses, for example, which is applied to the substrate by adhesive bonding or lamination. The article “Organic light emitting device with an ordered monolayer of silica microspheres as a scattering medium” by T. Yamasaki et al., Appl. Phys. Lett., Volume 76, page 1243 (2000), proposes applying microspheres made of quartz glass to the surface of the substrate in order to improve the coupling-out of light in the case of an OLED. Said microspheres may also be arranged beside the OLED. Furthermore, it is also known to produce periodic strictures having a period length in the region of the wavelength of the light emitted by the OLED between substrate and first electrode, said periodic structure continuing into the optically active layer of the light-emitting diode. The geometry specified ultimately results in Bragg scattering that increases the efficiency of the component, see J. M. Lupton et al., Appl. Phys. Lett., Vol. 77, page 3340 (2000). The published German patent application DE 101 64 016 A1 furthermore relates to an organic light-emitting diode in which the at least one organic layer has different partial regions having different refractive indices. On account of the deflection at the phase boundaries within the organic portion, fewer photons remain captured as a result of wave-guiding losses in the layer than in the case of homogeneous layers.
In addition to this utilization of intrinsic inhomogeneities in the active organic layer, it is furthermore known to introduce impurities such as nanoparticles to the organic electroluminescent material, thus making it possible to avoid waveguide effects within the organic portion, see for example “Enhanced luminance in polymer composite light emitting devices”, by S. A. Carter et al., Appl. Phys. Lett., Vol. 71 (1997). Said nanoparticles may be composed for example of TiO2, SiO2 or Al2O3 and be embedded in a polymeric emitter material, such as MEH-PPV.
In addition to bottom emitting OLEDs, top emitting OLEDs are increasingly gaining in relevance since they have advantages over the former for specific applications. If both the two electrodes and the substrate are transparent, it is possible to provide an electroluminescent component which emits in its entirety, that is to say towards the top and bottom. If the substrate does not have to be transparent as in a top emitting OLED, besides glass it is also possible to use many other substrates which make it possible, for example, for the component to be flexible, that is to say pliable. Furthermore, metal foils, silicon wafers or other substrates with silicon-based electronic components and also printed circuit boards may also serve as substrates in a top emitting electroluminescent component of this type.